*Case 1:*
x g()
{
x b; *// Constructor*
b.print();
return b; *// Optimized away*
}
int main()
{
x b; *// Constructor*
x t=g(); *// Optimized away*
}
*Case 2:*
x g(x b) *// Copy constructor*
{
b.print();
return b; *// Return by value & destructor*
}
int main()
{
x b; *// Constructor*
x t=g(b); *// Pass by value and return by value*
}
This has to do with optimization of local objects done by the compiler.
Since in case 1 a local was to created/returned to initialize another local,
so what the compiler does is that it creates object 'b' at a memory location
where object t was to be created. (You may think of the compiler internally
using placement new)
Different compilers have different techniques of optimization.
Refer http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.10
And as per the above link this should not work on MS C++.NET
Regards,
Sandeep Jain
On Wed, Jul 13, 2011 at 10:35 AM, pawan yadav <[email protected]>wrote:
> @rahul thanks for reply
> o/p (of 1st program) :
> constructor
> constructor
> print statement
> destructor
> destructor
>
>
> here copy constructor is not called ... that's why i have posted this
> program
>
> On Tue, Jul 12, 2011 at 10:42 PM, rahul <[email protected]> wrote:
>
>> Hi,
>>
>> In first program,it will call copy constructor once, while returning
>> object by value.
>> In 2nd program it will call copy constructor twice, once when u pass an
>> object to function by value, and 2nd time when you return an object by value
>> from function.
>>
>> Thanks.
>>
>> On Tue, Jul 12, 2011 at 11:52 PM, segfault <[email protected]>wrote:
>>
>>> #include<iostream>
>>> using namespace std;
>>> class x{
>>> int p;
>>> public:
>>> x(){cout<<"constructor\n";}
>>> x(const x &y)
>>> {
>>> cout<<"copy constructor\n";
>>> }
>>> ~x()
>>> {
>>> cout<<" destructor\n";
>>> }
>>> void print()
>>> {
>>> cout<<"print statement\n";
>>> }
>>> };
>>> x g()
>>> {
>>> x b;
>>> b.print();
>>> return b;
>>> }
>>> int main()
>>> {
>>> x b;
>>> x t=g();
>>> }
>>>
>>>
>>>
>>> #include<iostream>
>>> using namespace std;
>>> class x{
>>> int p;
>>> public:
>>> x(){cout<<"constructor\n";}
>>> x(const x &y)
>>> {
>>> cout<<"copy constructor\n";
>>> }
>>> ~x()
>>> {
>>> cout<<" destructor\n";
>>> }
>>> void print()
>>> {
>>> cout<<"print statement\n";
>>> }
>>> };
>>> x g(x b)
>>> {
>>>
>>> b.print();
>>> return b;
>>> }
>>> int main()
>>> {
>>> x b;
>>> x t=g(b);
>>> }
>>>
>>>
>>> why first one is not calling copy constructor in function g() while
>>> returning from it
>>> but second one is calling copy constructor in function g() while
>>> returning from it?
>>>
>>> in both program inside g() b is local but why giving different result.
>>>
>>> bruce ackel page number:467
>>>
>>>
>>>
>>> please explain it.
>>>
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